1. Field of the Invention
The present invention generally relates to a voltage generating apparatus.
2. Description of Related Art
With the popularization of electronic products, the electronic products are promoted all over the world. It is the most basic requirement that the same kind of electronic products should be able to work in completely different environments. For example, the same type of mobile phone may be sold to high-latitude countries with cold weather, or sold to countries on the hot equator. Further, due to the mobility of the user, the same mobile phone must work in different environments. To meet the above practical demands, it is a critical issue for designers to provide a circuit adaptable to changes of the environment.
In all the electronic systems, some analog circuits are indispensable. These analog circuits generally require an accurate reference power supply to remain stable. Thus, many so-called band gap voltage generating apparatus are put forward. The most important achievement of the voltage generating apparatus is the self-compensation capability of the output voltage confronted with a changing temperature. FIG. 1 shows a conventional voltage generating apparatus with temperature compensation capability. In this conventional voltage generating apparatus, two bipolar junction transistors (BJTs) Q1, Q2 are adopted, in which the current on a collector of each BJT rises when the temperature is increasing (i.e., a positive temperature coefficient (PTC)), so as to compensate the drop of the span-voltage between an emitter and a base of each BJT due to the increase of the temperature (i.e., a negative temperature coefficient (NTC)), thereby maintaining an output voltage VREF.
However, besides to output an accurate and stable voltage, the power consumption of the circuit should also be considered. In the conventional apparatus shown in FIG. 1, due to a restrained input voltage, an operational amplifier U1 needs a high system voltage to work normally, and thus the voltage generating apparatus has to consume a large amount of power. Therefore, to resolve this, the architecture of another conventional voltage generating apparatus is proposed, as shown in FIG. 2. In the conventional voltage generating apparatus of FIG. 2, a resistor string is employed to divide the input voltage of the operational amplifier U1 in FIG. 1, and then the voltage is input into the operational amplifier U1 accompanied with a new input circuit of the operational amplifier U1 (the input circuit is only constituted by metal oxide semiconductor field effect transistors (MOSFETs)), so as to lower the working voltage of the operational amplifier U1, thereby decrease the power consumption. Moreover, as a new output stage circuit is added, such a conventional voltage generating apparatus may output an output voltage VREF lower than 1 V.
FIGS. 3 and 4 show the architecture of another conventional voltage generating apparatus. Different from the above conventional voltage generating apparatus, the voltage generating apparatus in FIGS. 3 and 4 are constituted by complementary metal oxide semiconductor field effect transistors (CMOSFETs). This conventional circuit architecture has the advantages that the adopted CMOSFETs are cheaper, and it is easy to output an output voltage VREF lower than 1 V compared with the above circuit with BJTs architecture using the CMOSFETs.